The present invention relates to refractometers and more particularly to airborne types of microwave refractometers to obtain an instantaneous and continuous index of refraction of the atmosphere through which the aircraft bearing the refractometer is flying to evaluate the microwave energy scattering occasioned by these changes. Changes in the index of refraction of the atmosphere can be compensated in such equipment as radio and radar transmitters and receivers, and the like.
In the 1950s the method of measuring refractive index profiles of the atmosphere employed complex vacuum tube devices requiring highly skilled operators but these devices were not suitable for general usage. In the early 1960s refractometer devices were developed with solid state elements therein. One such solid-state refractometer was developed and disclosed in U.S. Pat. No. 3,356,941 which exceeded the capabilities of the tube devices but had the disadvantage of requiring a long warm-up period prior to use and also required good temperature control to obtain satisfactory results.
In U.S. Pat. No. 3,601,695, which issued Aug. 24, 1971, to the present inventor, there is described a refractometer which did not require any warm-up time. In this device the oscillations of a temperature compensated crystal controlled oscillator and a sweep oscillator are mixed and the difference frequency filtered into two intermediate frequency (IF) channels. One channel provides an automatic frequency control (AFC) for the sweep oscillator while the other channel provides automatic bandwidth control (ABC) for the sweep oscillator thereby eliminating the necessity for oven controlled circuit temperature. This precisely controlled sweep frequency is applied to a transmission type sampling cavity exposed to the atmosphere. When a resonant frequency is encountered, an electrical signal is developed and converted to a DC signal representative of the refractive index of the atmosphere. The disadvantage of this patented device is that the use of two microwave frequency sources are required which add considerably to the size, cost and power consumption of the refractometer.